Tyrosine kinase is a kind of proteins with tyrosine kinase activity, and can catalyze the transferring of phosphoric acid groups from ATPs to tyrosine residues of many important proteins to cause phosphorylation of these important proteins such that the downstream signal transduction pathways are activated. Protein tyrosine kinase plays a very important role in the intracellular signal transduction pathways, it regulates a series of physiological and biochemical processes such as growth, differentiation and death of cell bodies. Protein tyrosine kinase dysfunction may cause a series of diseases including tumors and eye diseases.
The occurrence, development and metastasis of many tumors as well as formation of tumor neovascularization have an extremely close relationship with abnormal expressions of tyrosine kinase. Especially, some tyrosine kinase receptors have abnormal expression in solid tumors, wherein vascular endothelial cell growth factor receptors (VEGFRs) present high expression in many tumor cells and tumor vascular endothelial cells, and platelet-derived growth factor receptors (PDGFRs) have abnormal expression in tumor stromal fibroblasts. The autocrine loop formed by ligands and receptors of tyrosine kinase directly participate in the occurrence and development of tumor cells, for example, vascular endothelial cell growth factor receptor (VEGFRs) exists in melanoma; platelet-derived growth factor receptors (PDGFRs) are present in gliomas; and stem cell growth factor receptors (KIT) resides in small cell lung cancer, and the like. Besides, similar loop also exists in meningiomas, neuroendocrine tumors, ovarian cancer, prostate cancer and pancreatic cancer. This loop has a very close relationship with the occurrence and development of tumors.
Moreover, the occurrence, development and metastasis of solid tumors depend on formation of tumor neovascularization which provides essential nutrients and oxygen for the growth of tumor. Tumor angiogenesis is an important process for invasion, migration and proliferation of tumor cells. Vascular endothelial growth factor receptor (VEGFR) family and platelet derived growth factor receptor (PDGFR) family are directly related to the occurrence and development of tumor and formation of tumor angiogenesis. Vascular endothelial growth factor (VEGF) known as the most powerful vascular penetrating agent and endothelial cell specific mitogen, plays an important role in the proliferation, migration and angiogenesis of endothelial cells. The expression level of vascular endothelial growth factor (VEGF) and the vascularization degree of tumor tissue show a significant positive correlation. VEGF mainly acts on high affinity receptors VEGFR-1 and KDR in endothelial cells which have different signal transduction pathways such that tyrosine kinase is phosphorylated to play its biological action. KDR plays a key role in growth, metastasis and angiogenesis of tumor. Platelet derived growth factor (PDGF) and its receptor (PDGFR) involves with the pathogenesis of multiple tumors and play important roles in angiogenesis. Platelet derived growth factor (PDGF) shows its cell biological effect via its receptor (PDGFR). PDGFR maintains the integrality of vascular wall and promotes the formation of tumor neovascularization by regulating the proliferation and migration of vascular wall perithelial cells and vascular smooth muscle cells. In addition, the growth of tumor is promoted by changing the microenvironment within tumor.
Due to that the abnormal expression of tyrosine kinase is very closely related to the occurrence, development and metastasis of tumor as well as formation of tumor neovascularization, drug research and development targeting tyrosine kinase has become a focus of international research of anti-tumor drugs. In particular, it is a new strategy to treat cancer that targeting neovascularization to inhibit the formation of tumor angiogenesis and block the nutrition supply and migration path of tumor to prevent the growth and metastasis of tumor. The abnormal expression of KDR or PDGFR receptors plays a key role in the formation of tumor neovascularization, and therefore KDR and PDGFR receptors have become the most ideal target for anticancer drug therapy. Moreover, two anti-tumor drugs Sorafenib and Sunitinib (SU11248) mainly inhibiting KDR and PDGFR receptor tyrosine kinases, approved by the US Food and Drug Administration (FDA), have fully demonstrated their anti-tumor therapeutic effect with high curative effect and fewer side effects in clinical practice.
Histone deacetylase (HDACs) is a kind of metalloprotease, which plays a key role in chromosome structure modification and gene expression modulation. In cancer cells, overexpression of HDACs leads to increasing of binding force between histone and DNA such that abnormally allosteric chromosome occurs. At the same time, expression of cell cycle inhibitory factor is inhibited, and stability and binding capacity with DNA of tumor suppressor gene p53 decrease, whereas expression of hypoxia inducible factor-1 (HIF-1) and vascular endothelial growth factor (VEGF) increases. In mammalian cells, the balance between acetylation and deacetylation plays a key role in gene transcription and the function of different cellular proteins. Acetylation of histone is regulated by histone acetyltransferases (HATs) and histone deacetylase (HDACs). The dynamic balance between HDACs and HATs controls the structure of chromatin and expression of genes, and the dysfunction thereof is one of the important molecular mechanisms of tumor development. HDACs belong to the super family of deacetylases, and have four types, type I, II, III and IV. At present, type I HDACs is most studied. Type I HDACs includes HDAC1, HDAC2, HDAC3 and HDAC8. HDACs play an important role in chromatin remodeling, gene repression, regulation of cell cycle and differentiation. Dysfunction of histone deacetylase in tumor cell may lead to gene transcription inhibition and inhibit expression of cancer suppressor genes. High expression of HDAC1 in tumor cells can significantly increase the proliferation ability of tumor cells, and high expression of HDAC1 can affect the extracellular matrix such that migration and invasiveness of tumor cells is enhanced significantly. It is reported that HDAC1mRNA levels and protein levels are highly expressed in gastric cancer tissues, prostate cancer tissues, colon cancer tissues and liver cancer tissues, and are correlated with TNM stage and lymph node metastasis of tumors. It is found by study that inhibiting HDACs activity can effectively inhibit cancer cell proliferation, induce cell cycle arrest and promote cell apoptosis, and therefore, HDACs becomes a new target of anticancer drug design, and developing HDAC inhibitor (HDACi) is considered as an effective strategy for cancer therapy. Micromolecule HDACi of hydroxamic acids is a kind of HDACi been paid much attention in recent years, which demonstrates a good anti-tumor activity both in vivo and in vitro. In 2006, the first hydroxy acid HDACi, SAHA (generic name: Vorinostsat; commodity names: Zolinza, ) approved by the United States FDA is used for treating skin T cell lymphoma. Subsequently, more and more HDACi enter into the clinical as treatment and adjuvant therapy for solid tumors such as colon cancer, lung cancer and the like, and malignancies of the blood system such as leukemia and lymphoma, etc.
It is also found in recent studies that combined utilization of targeted drugs has a better effect. Two drugs with different targets may work better in the same combined drugs.
In a study disclosed in Cancer Research (Qian D Z, Wang X, Kachhap S K, Kato Y, Wei Y, Zhang L, et al. The histone deacetylase inhibitor nvp-laq824 inhibits angiogenesis and has a greater antitumor effect in combination with the vascular endothelial growth factor receptor tyrosine kinase inhibitor ptk787/zk222584; Cancer Res 2004; 64:6626-34), such combination effect was proved in a mouse and cell lines models by researchers. These preliminary results showed very good prospects: the two different drugs target the development process of cancers in these two kinds of cells in a manner of “continuous strike”.
Researcher of Johns Hopkins Kimmel Cancer Center believed that the single use of anti angiogenesis drugs in human clinical researches is difficult to achieve the desired effect on the target tumor. Previous studies have indicated that a kind of drugs that can normalize DNA winding may also affect the growth of blood vessels. Dr. Roberto Pili suggests that combination of these two kinds of drugs may have a greater impact on the development of cancers.
An anti-angiogenic drug called PTK787/ZK222584 was chosen by researchers to carry out the research, and this drug can inhibit the function of VEGF (vascular endothelial growth factor) proteins which may lead to cascade reactions of cell signal promoting angiogenesis. Such VEGF inhibitors and histone deacetylation (HDAC) inhibitors are used in combination. Cancer cells can take the acetyl groups away from histones resulting in that DNAs wrap together all the time, and therefore the activation of genes is inhibited. By utilizing histone deacetylation (HDAC) inhibitors to block the function of displacing the acetyl groups of HDACs, this error may be reversed and it is able to unfold DNAs and produce the desired gene product.
It is found in the research that the combination use of VEGF inhibitors and HDAC inhibitors NVP-LAQ824 may achieve 51% inhibition of cultured endothelial cells (that is twice of the effect when using the two drugs alone). In mouse models, this drug combination may induce 60% inhibition of new blood vessel formation, whereas the single use may induce 50% inhibition. The tumor growth inhibition ratios in the mouse suffered from a prostate cancer are 35 and 75%, respectively. This inhibition ratio is 85% when combination use. The two inhibitors respectively exhibited 54% and 60% growth inhibitions on tumors of mouse suffered from a breast cancer, whereas combination treatment showed 80% inhibition on tumor growth.
Combination medication of anti-VEFG monoclonal antibody Bevacizumab and HDAC inhibitor Valproic ACI combination has been carried out in Phase I clinical trials to provide synergistic effects and enhance drug efficacies for the treatment of intestinal cancer, gastric cancer and prostate cancer (J Clin Oncol 29: 2011), which showed better curative effects than single medication.
As mentioned above, the combination of drugs with different targets produces a better curative effect than single medication; however, there are still problems in hybrid applications of different drugs due to their different properties and metabolisms, thereby affecting the effects of the drugs.